1. Field of the Invention
The present invention relates to a novel catalytic material useful for the polymerization of ethylene and a process for polymerizing ethylene using this catalytic material.
2. Description of the Prior Art
The polymerization of olefins, such as ethylene and propylene, using the so-called Ziegler-type catalysts, has been known for some time. These Ziegler-type catalysts are defined, for example, in U.S. Pat. No. 3,989,881, as combinations of transition metal compounds of Groups IVB-VIB of the Periodic Table and organometallic compounds of Groups IA-IIIA of the Periodic Table. However, many of the conventional catalysts do not possess satisfactorily high catalytic activity for the production of polyethylene and thus it is difficult to utilize the resulting polymers without removal of catalyst residue following polymerization. In addition, when polyethylene is prepared by the so-called liquid phase slurry type polymerization, the bulk density of the resulting polyethylene powder is frequently less than desired.
Moreover in the process described in the prior art, transition metal compounds are contacted with solid carriers which limits the ability of the transition metal compounds to be evenly distributed across the surface of the solid carrier.
With some inert catalyst carriers such as silica, the number and distribution of sites available for complexation to the transition metal is difficult to control. For example, it is well known to those skilled in the art that when silica is used as a carrier and the number of sites is decreased by a process such as calcination, a clustering of the remaining sites occurs which further compounds the problem. The use of silanols allows for distribution of the sites to be "dialed into" the catalyst carrier. This allows more sites per unit of surface area and resultantly more activity.
Additionally, most prior polyethylene catalysts have poor sensitivity to hydrogen; that is, they require a large amount of H.sub.2 with ethylene (up to a molar ratio of 1:1) on order to effect significant changes in polymer chain length, and hence, melt index. These silanol-based catalysts demonstrate a suprising sensitivity to H.sub.2 with the MI varying over a substantial range with relatively small changes in H.sub.2 pressure. This is an advantage due to ease of processability and wide flexibility in the type of polyethylenes which can be made.